Pigment

ABSTRACT

A mixed clay/calcium carbonate pigment wherein the clay pigment component has associated therewith before being mixed with the calcium carbonate pigment component a water-soluble condensed phosphate dispersing agent and a second dispersing agent which is a water-soluble salt comprising an organic macromolecular anion, for example a salt of polyacrylic or polymethacrylic acid, and wherein the calcium carbonate pigment component has associated therewith before being mixed with the clay pigment component said second dispersing agent.

Umted States Patent [151 3,661,610 Ferris 1 May 9, 1972 [5 PIGMENT [56]References Cited [72] Inventor: Andrew Paul Ferris, Cornwall, EnglandUNITED STATES PATENTS 3,085,894 4/1963 Rowland ..106/306 [73] Assgneei PLwermg 2,540,182 2 1951 Albert 106/308 B Pany Llmlted Cornwall England3,130,063 4/1964 Millman et a1. ..l06/288 B [22] Filed: Sept. 8, 1970Primary E.\'aminer.lames E. Poer [21] Appl. No.: 70,104Attorney-Cushman,Darby&Cushman [57] ABSTRACT [30] Foreign ApplicationPriority Data A mixed clay/calcium carbonate pigment wherein the claySept. 10, Great Britain pigment component has asso iated therewithbefore being mixed with the calcium carbonate pigment component a [52]U.S.CI. .106/306, 106/288 B, 106/308 B, Water-soluble condensedphosphate dispersing agent and a 10 /30 Q 106 309 second dispersingagent which is a water-soluble salt compris- 51 1m. 01 ..C09c 1 02, CO9C1/28 ing an Organic macromolecular anion, for example a Sail of 58 Fieldof Search ..l06/72 288 B 306 308 B Pmyzcrylic Pdymethacryli acid andwherein the Calcium carbonate pigment component has associated therewithbefore being mixed with the clay pigment component said seconddispersing agent.

16 Claims, 1 Drawing Figure PIGMENT This invention relates to pigmentsand, more particularly, is concerned with a mixed pigment suitable foruse in, for example, paper coating compositions, paints, rubbers andplastics materials.

It is known to produce a deflocculated aqueous suspension of a claypigment, for example a kaolinitic clay, and to spraydry thedeflocculated aqueous suspension of clay pigment in order to obtainsmall spherical particles of the pigment which are free-flowing and areeasy to redisperse in water for incorporation in, for example, a papercoating composition, such a material being known as a predispersedpigment. The dispersing agents generally used for forming thedeflocculated aqueous suspension of clay pigment are water-solublecondensed phosphates, for example tetrasodium pyrophosphate, sodiumhexametaphosphate and sodium tripolyphosphate. It is also known to treata calcium carbonate pigment, for example chalk whiting, in a similarmanner but using as the dispersing agent a water-soluble salt comprisingan organic macromolecular anion, for example a sodium polyacrylate or asodium polymethacrylate, a sodium salt of a sulphonatednaphthalene-formaldehyde condensation product or a sodiumlignosulphonate.

It is also known to prepare a mixture of a kaolinitic clay pigment and acalcium carbonate pigment, the mixed pigment thus obtained combining thegood optical properties of the kaolinitic clay pigment with the goodrheological properties of the calcium carbonate pigment. It is found,however, that if aqueous suspensions of the two components of the mixedpigment are deflocculated in a conventional manner, i.e. the kaoliniticclay pigment with a water-soluble condensed phosphate dispersing agentand the calcium carbonate pigment with a dispersing agent which is awater-soluble salt comprising an organic macromolecular anion, such as asodium polyacrylate or a sodium polymethacrylate, and the two aqueoussuspensions then blended together, there is obtained a deflocculatedsuspension of the mixed pigment the rheological properties of whichdeteriorate seriously on storage. This means that heretofore a papermanufacturer, for example, has had to prepare the mixed pigment as andwhen required, and has not been able to obtain a suitable stablesuspension of a mixed pigment or a predispersed mixed pigment from apigment manufacturer.

It is an object of the present invention to provide a mixed pigmentwhich forms a stable suspension. 5

SUMMARY OF THE INVENTION According to one aspect of the presentinvention there is provided a mixed pigment comprising a clay pigmentcomponent and a calcium carbonate pigment component, wherein the claypigment component has associated therewith before being mixed with saidcalcium carbonate pigment component a quantity of a first dispersingagent which is a water-soluble condensed phosphate and a quantity of asecond dispersing agent which is a water-soluble salt comprising anorganic macromolecular anion, and wherein the calcium carbonate pigmentcomponent has associated therewith before being mixed with said claypigment component a quantity of said second dispersing agent.

According to another aspect of the invention there is provided a methodof preparing a mixed pigment which method comprises the steps of (a)forming an aqueous suspension of a clay pigment containing a quantity ofa first dispersing agent which is a water-soluble condensed phosphateand a quantity of a second dispersing agent which is water-soluble saltcomprising an organic macromolecular anion, (b) forming an aqueoussuspension of a calcium carbonate pigment containing a quantity of saidsecond dispersing agent, and (c) blending together the aqueoussuspensions of the clay pigment and of the calcium carbonate pigment.

In one embodiment of the method according to the invention, the blendedsuspension is then dried, for example by means of a spray drier, a drumdrier or a fluidized bed drier, to form a predispersed mixed pigment. Analternative method of obtaining a predispersed mixed pigment is to drythe aqueous suspensions of the clay pigment and of the calcium carbonatepigment separately, and then blend together the dried pigmentcomponents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably, the calciumcarbonate pigment component also has associated therewith before beingmixed with said clay pigment component a quantity of said firstdispersing agent.

The first dispersing agent is a water-soluble condensed phosphate andcan be, for example, tetrasodium pyrophosphate, sodium hexametaphosphateor sodium tripolyphosphate.

The second dispersing agent is a water-soluble salt comprising anorganic macromolecular anion which preferably has a number averagemolecular weight in the range to 10,000. The organic macromolecularanion can be polymeric and examples of suitable water-soluble salts ofthis type include sodium, potassium or ammonium salts of polyacrylicacids or of polymethacrylic acids, and sodium salts of the sulphonatedcondensation products of naphthalene and formaldehyde. Otherwater-soluble salts having organic macromolecular anions with a numberaverage molecular weight in the range 100 to 10,000 which can be usedinclude water-soluble salts of lignosulphonic acid.

The first and second dispersing agents used with each pigment componentcan be the same or different.

The amount of the first dispersing agent, viz. the water-solublecondensed phosphate, associated with the clay pigment component ispreferably in the range of from 0.1 to 0.5 percent by weight, mostpreferably in the range of from 0.15 to 0.40 percent by weight, based onthe weight of dry clay pigment, and the amount of the second dispersingagent, viz. the water-soluble salt comprising an organic macromolecularanion, associated with the clay pigment component is preferably in therange from 0.01 to 0.30 percent by weight, most preferably in the rangefrom 0.02 to 0.20 percent by weight, based on the weight of dry claypigment.

The amount, if any, of the first dispersing agent, viz. thewater-soluble condensed phosphate associated with the calcium carbonatepigment is preferably in the range of from 0.02 to 0.30 percent byweight, most preferably in the range from 0.05 to 0.20 percent byweight, based on the weight of dry calcium carbonate pigment, and theamount of the second dispersing agent, viz. the water-soluble saltcomprising an organic macromolecular anion, associated with the calciumcarbonate pigment is preferably in the range from 0.01 to 0.30 percentby weight, most preferably in the range from 0.02 to 0.20 percent byweight, based on the weight of dry calcium carbonate.

The calcium carbonate pigment can be a natural chalk whiting, groundmarble or calcite, or a chemically-precipitated calcium carbonatepigment, and should generally consist substantially completely ofparticles passing a No. 300 mesh B.S. sieve. Advantageously, there isused a natural chalk whiting in which case it is preferably ground untilit has a particle size distribution such that at least 65 percent byweight thereof consists of particles smaller than 2 microns equivalentspherical diameter, not more than 10 percent by weight thereof shouldconsist of particles larger than 10 microns equivalent sphericaldiameter, and not more than 1 percent by weight thereof consists ofparticles larger than 53 microns. The grinding of the natural chalkwhiting can conveniently be performed in two stages: in the first stageit is subjected to a light grinding in a pebble mill, for a period ofabout 10-30 minutes, in the presence of water containing a quantity ofthe second dispersing agent in order to separate the hard impurities,e.g. flint, from the fluid suspension, the pebbles used in this first 3stage advantageously having sizes which are in the range of from to 100mm.; and in the second stage the grinding is completed in the presenceof the second dispersing agent in a second pebble mill containing aparticulate grinding medium consisting of particles having sizes in therange of from No.

100 mesh B.S. sieve to mm. for a time sufficient to reduce.

ample a kaolin, sometimes known as china clay. Other clay pigments whichcan be used include halloysite, dickite and nacrite. The clay pigmentshould preferably have a particle sizedistribution such that at least 50percent by weight thereof consists of particles smaller than 2 micronsequivalent spheri- I cal diameter, not more than 15 percent by weightthereof consists of particles larger than 10 microns equivalentspherical diameter, and not more than 0.05 percent by weight thereofconsists of particles largerthan 53 microns. A suspension suitable formixing withthe calcium carbonate pigment suspension can be'prepared byfirst dewatering the clay and then mixing the cake thus obtained with anaqueous solution containing the requiredv amount of the first and seconddispersing agents in aliquid mixer, e.g. a blunger. The clay suspensioncan be dewatered by pressure filtering, for example in a tube pressurefilter of the type disclosed in Belgian Pat. specification No. 717,264or in recessed plate filter press, or by means of a vacuum filter or acentrifuge.

The suspension of clay pigment can be blended with the suspension ofcalcium carbonate pigment in a mixing vessel, for example an impellermixer. The order of addition of the two components to the mixing vesselis not important.

The blended aqueous suspension of the mixed pigment, whichadvantageously comprises from 60 to 80 percent by weight of solids, canbe transported as a stable slurry in suitable containers, or it can bedried in, for example, a spray drier in order to form a substantiallydry material, i.e. a predispersed mixed pigment, which can be readilyredispersed in water.

in the case of the predispersed mixed pigment, it has been found that inorder to achieve the lowest viscosity and the greatest rheologicalstability on storage of the redispersed mixed pigment it is advantageousto add, when redispersing the dry mixed pigment, further smallquantities of thefirst dispersing. agent andof the second dispersingagent. The amount of the first dispersing agent added at this stage ispreferably in the range of from 0.03 to 0.06 percent by weight,

based on the total weight of dry solids in the mixed pigment,

and the amount of the second dispersing agent added at this stage ispreferably in the range 0.02 to 0.04 percent by weight, based on thetotal weight of dry solids. It has also been found that the amountsoffirst-and second dispersing agents added to the mixed pigment whenredispersing the same are preferably such that they are in theproportions of about 3 parts by weightof the first dispersing agent toabout 2 parts by weight of the second dispersing agent.

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawing which shows one arrangement of aninstallation suitable for carrying out the invention.

Referring to the drawing, raw natural chalk whiting (W), a sodiumpolyacrylate dispersing agent (D and water are introduced into a pebblemill 1 containing pebbles ranging in size from 50 to 100 mm. The mixtureis ground for 15 minutes so that the particles of chalk whiting passinto suspensior'ileaving the hard impurities, e.g. flint. The suspensionis then passed through a series of three sieves 2, 3 and 4, of 'whichsieve 2 has a nominal aperture of V4 inch, sieve-3 is a No. 10 mesh B.S.sieve and sieve 4 is a No. 60 mesh B.S. sieve. The screened suspensionof chalk whiting is then further ground in a sand grinding mill 5 with agrinding medium consisting of ,ting consists of particles smaller than 2microns equivalent spherical diameter. The ground suspension is thenpassed through two further sieves 6 and 7 of which sieve 6 is a No. 100mesh B.S. sieve and sieve 7 a No. 300 mesh B.S. sieve to a mixing vessel8 in which the required proportion of tetrasodium.pyrophosphatedispersing agent (D is added. The fully deflocculat'ed aqueoussuspension of chalk whiting is transferred, by means of apump 9, to asecond mixing vessel 10 in which it is blended with a fullydeflocculated suspension of a kaolin clay.

The deflocculated suspension of kaolin clay is formed by dewatering asuspension of a fine kaolin clay (C) having a particle size distributionsuch that at least 50 percent by weight thereof consists ofparticles'smaller than 2 micronsequivalent spherical diameter and notmore than 0.05 percent by weight thereof consists of particles largerthan 53 microns, in a recessed plate filter-press 11 to form a filtercake containing 27 percent to 30 percent by weight of water. This filtercake is mixed in a blunger 12 with an aqueous solution of the requiredquantities of sodium polyacrylate (D and tetrasodium pyrophosphate (Dand the resultant fully deflocculated suspension of kaolin clay is mixedwith the chalk whiting suspension in the mixing vessel 10.

The blended suspension of mixed pigment can either be transferred totanker vehicles 13, for transport as a stable suspension, or dried in aspray-drier 14. In the latter case, the gas leaving the spray-drier 14is passed through a cyclone 15 in order to separate from it entrainedsolid particles which are added to the dry product from the spray-drier.The gas leaving the cyclone is finally passed through a bag filter (notshown) before being exhausted to atmosphere.

The invention is further illustrated by the following examples.

EXAMPLES l ble mill containing, as grinding medium, pebbles ranging insize from 10 to 35 mm. together with 2.27 liters of an aqueousdispersing agent which is a sodium salt of a polyacrylic acid having anumber average molecular weight of 1,650, the quantity of the dispersingatent added being0.l percent by weight based on the weight of dry chalkwhiting. Theraw natural chalk whiting contained 20 percent by weight ofwater and the solids content of the mixture in the pebble mill wastherefore 76 percent by weight. The mixture was ground for a total timeof 18 hours after which the aqueous suspension was screened successivelythrough a No. 60 meshB.S. sieve, a No. mesh B.S. sieve, and a No. 300mesh B.S. sieve. The ground chalk whiting passing the last sieve wasfound to have a particle size distribution suchthat 70 percent by weightthereof consisted of particles smaller than 2 microns equivalentspherical diameter. A quantity of tetrasodium pyrophosphate dispersingagent, equivalent to 0. 15' percent by. weight based on the weight ofdry chalk whiting, was then addedto the suspension with stirring and theresultant suspension was identified as W A second suspension wasprepared from the same raw natural chalk whiting in an identical mannerto that described above except that, instead of the addition. oftetrasodium pyrophosphate dispersing agent, a second addition was madeof the sodium polyacrylate dispersing agent amounting to 0.1 percent byweight based on the weight of dry chalk whiting, thus making a totaldispersing agent addition of 0.2 percent by weight of sodiumpolyacrylate based on the weight of dry chalk whiting. This second chalkwhiting suspension was identified as W v A suspension of a fine kaolinclay having a particle'size distribution'such that 81 percent byweightthereof consisted of particles smaller than 2 microns equivalentspherical diameter was prepared by mixing 36.6 kg of dry kaolin clay ina tank with 19.5 liters of water containing sodium polyacrylate andtetrasodium pyrophosphate dispersing agents, and sufficient sodiumhydroxide to give the liquor a final pH of about 7. The suspensioncontained 0.05 percent by weight of sodium polyacrylate dispersing agentand 0.25 percent by weight of tetrasodium pyrophosphate, based on theweight of dry kaolin clay, and the solids content of the suspension was65 percent by weight. This suspension was identified as C,.

A further suspension was prepared from the same fine kaolin clay in anidentical manner except that no sodium polyacrylate dispersing agent wasadded and the quantity of tetrasodium pyrophosphate dispersing agent was0.2 percent by weight based on the weight of dry kaolin clay. The finalpH was again adjusted to about 7 with sodium hydroxide. This suspensionwas identified as C,..

Each chalk whiting suspension was blended with each kaolin claysuspension in proportions such that the weight of dry chalk whitingpresent was equal to the weight of dry kaolin clay. The blending of thesuspensions was achieved by mixing the two suspensions in a tank forabout 10 minutes, during which time about 5 horsepower hours of energyper ton of solids were dissipated in the blended suspensions. The fourblended suspensions comprised, respectively, W, C,, W, C W C, and W Cand each suspension contained 70 percent by weight ofsolids.

Samples of the four blended suspensions were tested for rheologicalstability by measuring their viscosities at 22 C. using a BrookfieldViscometer at speed 100 r.p.m. at 0, 2, 15, 24, 96 and 200 hours,respectively, after mixing. The results obtained are shown in table Ibelow.

These results show that the combination of either suspension W, or Wwith suspension C,, i.e. the combination in which the kaolin clay hadbeen deflocculated with sodium polyacrylate and tetrasodiumpyrophosphate, and the chalk whiting had been deflocculated with atleast the sodium polyacrylate, gave the most stable blends.

The composition W, C, was then diluted to 63 percent by weight of solidsand dried in a conventional spray drier. The dried material was thendivided into four portions A, B, C and D of which was mixed with waterto form a suspension containing 70 percent by weight of solids. Toportion A no extra deflocculant was added; to portion B there was added0.10 percent by weight of sodium polyacrylate dispersing agent based onthe weight of dry solids; to portion C there was added 0.15 percent byweight of tetrasodium pyrophosphate based on the weight of dry solids;and to portion D there were added 0.030 percent by weight of sodiumpolyacrylate dispersing agent and 0.045 percent by weight of tetrasodiumpyrophosphate based on the weight of dry solids. The viscosities at 22C. of the four suspensions at 0, 50, 100, 150 and 200 hours,respectively, after mixing were measured using a Brookfield Viscometerat speed 100 r.p.m. The results obtained are shown in table 11 below.

TABLE II Time (hours) 0 50 100 150 200 Viscosity at 22C. Portion(centipoise) These results show that the suspension with the bestrheologi cal properties for the purpose of forming, for example, a papercoating composition is obtained by adding small additional quantities ofboth sodium polyacrylate and tetrasodium pyrophosphate to theredispersed spray dried material.

EXAMPLE 2 The product manufactured by spray drying the blendedsuspension W, C, was used as extender in an emulsion paint compositionsuitable for external use which had the following formulation:

Water to 82 Krebs Units viscosity The pigment volume concentration ofthe formulation was 40 percent. The dispersant consisted of a mixture of5 parts by weight of sodium hexametaphosphate, 4 parts by weight ofsodium hydroxide and 1 part by weight ofsodium polyacrylate dispersingagent in a 5 percent w/v solution.

As a comparison a second emulsion paint composition was prepared inexactly the same way except that the kaolin/chalk whiting mixture wasreplaced by 104 parts by weight of a kaolin clay having a particle sizedistribution such that 50 percent by weight consisted of particlessmaller than 2 microns equivalent spherical diameter, 10 percent byweight of particles larger than 10 microns equivalent spherical diameterand 0.01 percent by weight of particles larger than 53 microns (No. 300mesh B.S. sieve).

Each composition was prepared by mixing the pigment and the extenderwith the dispersant, the fungicide and the defoaming agent for 2 minutesusing a high-speed, shrouded impeller mixer. The hydroxy-ethyl-cellulosewas then added and the composition mixed for a further minute. Finallythe polyvinyl acetate copolymer and the butyl Cellosolve acetate wereadded and the composition mixed for another minute. The paints wereallowed to stabilize overnight before their viscosity was adjusted to 82Krebs units.

Each paint was tested for gloss at angles of 45 and with the surface,opacity, brightness and brushability and flow, and the results are shownin table 111 below:

The gloss measurement was obtained by applying a film of each paint0.002 in. (0.05 mm.) in thickness with a draw bar applicator to a glassplate measuring 12 X 4 in. (305 X 102 mm.). The films were aged for 24hours before readings were pied to a G.E.C. brightness meter.

The opacity measurement was made by drawing out a film of each paint ofthickness 0.0032 in. (0.08 mm.) on a hiding power chart which wasdivided into black and white sections. The films were then allowed todry overnight and reflectance measurements were taken in duplicate overthe black and white portions of the charts. The apparatus used was areflectometer head fitted with a neutral density filter and coupled to aG.E.C. brightness meter. The apparatus was set to read 100 percent whenusing a freshly scraped magnesium carbonate block as a standard. Theaverage reflectance from the black and white portions were determinedand the ratio:

Average reflectance from the black portion y Average reflectance fromthe white portion was quoted as the opacity value.

The brightness measurement was made using the same charts and apparatusas the opacity measurement but the neutral density filter was replacedby a violet filter. The apparatus was again adjusted to give a scalereading of 100 when using the same magnesium carbonate block as astandard and the reflectance from the white portion of the chart wasmeasured for each paint film.

lclaim:

l. A mixed pigment consisting essentially of a clay pigment componentand a calcium carbonate pigment component, said clay pigment componenthaving associated therewith in the unmixed condition a quantity of afirst dispersing agent which is a water-soluble condensed phosphate anda quantity of a second dispersing agent which is a water-soluble saltcomprising an organic macromolecular anion, and said calcium carbonatepigment component having associated therewith in the unmixed condition aquantity of said second dispersing agent.

2. A mixed pigment as claimed in claim 1, wherein said calcium carbonatepigment component also has associated therewith in the unmixed conditiona quantity of said first dispersing agent. I

3. A mixed pigment as claimed in claim 1 wherein said first dispersingagent is selected from the group consisting of tetrasodiumpyrophosphate, sodium hexametaphosphate and sodium tripolyphosphate.

4. A mixed pigment as claimed in claim 1, wherein said second dispersingagent is a water-soluble salt having a polymeric anion with a numberaverage molecular weight in the range 100 to 10,000. i

5. A mixed pigment as claimed in claim 4, wherein said second dispersingagent is selected from the group consisting of the sodium, potassium andammonium salt of an acid selected from the group consisting of apolyacrylic acid and a polymethacrylic acid.

6. A mixed pigment as claimed in claim 1, wherein the amount of saidfirst dispensing agent associated with the clay pigment component is inthe range of from 0.1 to 0.5 percent by weight based on the weight ofdry clay pigment.

7. A mixed pigment as claimed in claim 1, wherein the amount of saidsecond dispersing agent associated with the clay pigment component isinthe range of from 0.1 to 0.30 percent by weight based on the weight ofdry clay pigment.

8. A mixed pigment as claimed in claim 1, wherein the amount of saidsecond dispersing agent associated with the calcium carbonate pigmentcomponent is in the range of from 0.01 to 0.30 percent by weight basedon the weight of dry calcium carbonate.

9. A mixed pigment as claimed in claim 1, wherein the calcium carbonatepigment component consists substantially completely of particles smallerthan 53 microns.

10. A mixed pigment as claimed in claim 1, wherein the clay pigmentcomponent is a kaolinitic clay.

11. A mixed pigment as claimed in claim 10, wherein said clay pigmentcomponent has a particle size distribution such that at least 50 percentby weight thereof consists of particles smaller than 2 micronsequivalent spherical diameter, not

more than 15 percent by weight thereof consists of particles larger than10 microns equivalent spherical diameter, and not more than 0.05 percentby weight thereof consists of particles larger than 53 microns.

12. A method of preparing a mixed pigment consisting essentially of aclay pigment component and a calcium carbonate pigment component saidmethod comprising the steps of (a) forming an aqueous suspension of theclay pigment component containing a quantity of a first dispersing agentwhich is a water-soluble condensed phosphate and a quantity of a seconddispersing agent which is a water-soluble salt comprising an organicmacromolecular anion, (b) forming an aqueous suspension of the calciumcarbonate pigment component containing a quantity of said seconddispersing agent, and (c) blending together the aqueous suspension ofthe clay pigment component and of the calcium carbonate pigmentcomponent.

13. A method according to claim 12, wherein the solids contents of saidaqueous suspension of a clay pigment component and said aqueoussuspension of a calcium carbonate pigment component are such that theblended aqueous suspension has a solids content in the range 60 topercent by weight.

14. A method according to claim 12, wherein after the aqueoussuspensions of the clay pigment component and of the calcium carbonatepigment component have been blended together the resulting blendingsuspension is spray-dried.

15. A method according to claim 12, wherein said calcium carbonatepigment component has associated therewith before being mixed with saidclay pigment component a quantity of said first dispersing agent.

16. A method according to claim 12, wherein the clay pigment componentis a kaolinitic clay.

2. A mixed pigment as claimed in claim 1, wherein said calcium carbonate pigment component also has associated therewith in the unmixed condition a quantity of said first dispersing agent.
 3. A mixed pigment as claimed in claim 1 wherein said first dispersing agent is selected from the group consisting of tetrasodium pyrophosphate, sodium hexametaphosphate and sodium tripolyphosphate.
 4. A mixed pigment as claimed in claim 1, wherein said second dispersing agent is a water-soluble salt having a polymeric anion with a number average molecular weight in the range 100 to 10,
 000. 5. A mixed pigment as claimed in claim 4, wherein said second dispersing agent is selected from the group consisting of the sodium, potassium and ammonium salt of an acid selected from the group consisting of a polyacrylic acid and a polymethacrylic acid.
 6. A mixed pigment as claimed in claim 1, wherein the amount of said first dispensing agent associated with the clay pigment component is in the range of from 0.1 to 0.5 percent by weight based on the weight of dry clay pigment.
 7. A mixed pigment as claimed in claim 1, wHerein the amount of said second dispersing agent associated with the clay pigment component is in the range of from 0.1 to 0.30 percent by weight based on the weight of dry clay pigment.
 8. A mixed pigment as claimed in claim 1, wherein the amount of said second dispersing agent associated with the calcium carbonate pigment component is in the range of from 0.01 to 0.30 percent by weight based on the weight of dry calcium carbonate.
 9. A mixed pigment as claimed in claim 1, wherein the calcium carbonate pigment component consists substantially completely of particles smaller than 53 microns.
 10. A mixed pigment as claimed in claim 1, wherein the clay pigment component is a kaolinitic clay.
 11. A mixed pigment as claimed in claim 10, wherein said clay pigment component has a particle size distribution such that at least 50 percent by weight thereof consists of particles smaller than 2 microns equivalent spherical diameter, not more than 15 percent by weight thereof consists of particles larger than 10 microns equivalent spherical diameter, and not more than 0.05 percent by weight thereof consists of particles larger than 53 microns.
 12. A method of preparing a mixed pigment consisting essentially of a clay pigment component and a calcium carbonate pigment component said method comprising the steps of (a) forming an aqueous suspension of the clay pigment component containing a quantity of a first dispersing agent which is a water-soluble condensed phosphate and a quantity of a second dispersing agent which is a water-soluble salt comprising an organic macromolecular anion, (b) forming an aqueous suspension of the calcium carbonate pigment component containing a quantity of said second dispersing agent, and (c) blending together the aqueous suspension of the clay pigment component and of the calcium carbonate pigment component.
 13. A method according to claim 12, wherein the solids contents of said aqueous suspension of a clay pigment component and said aqueous suspension of a calcium carbonate pigment component are such that the blended aqueous suspension has a solids content in the range 60 to 80 percent by weight.
 14. A method according to claim 12, wherein after the aqueous suspensions of the clay pigment component and of the calcium carbonate pigment component have been blended together the resulting blending suspension is spray-dried.
 15. A method according to claim 12, wherein said calcium carbonate pigment component has associated therewith before being mixed with said clay pigment component a quantity of said first dispersing agent.
 16. A method according to claim 12, wherein the clay pigment component is a kaolinitic clay. 